0309ctt volcano glass lo res

[Image above] Volcanic lightning about Chile’s Chaiten volcano. Credit: Paul Kim; Flickr CC BY-NC-SA 2.0

In addition to the ways man has figured out how to make glass, glass forms naturally—for example, from volcanic activity, meteorite impacts, and lightning strikes. All of these events can produce not just glass, but small glass spheres, or spherules. And it’s not an earth-specific phenomenon—glass beads are on the moon, too.

New research suggests that natural glass spheres also are born during another natural phenomenon—volcanic lightning.

Although there’s still a lot we don’t understand about this epic phenomenon, volcanic lightning, akin to thunderstorm lightning, is caused by separation of charged particles. However, during a volcanic eruption, the charges get separated when rock particles of the spewing volcanic ash forcefully rub together, generating friction.

In the new research, an international team of scientists studied volcanic ash deposits from two recent eruptions: Mount Redoubt, Alaska, in 2009 and Eyjafjallajökull, Iceland, in 2010. Both well-documented eruptions were accompanied by volcanic lightning.

The scientists had a ball sifting through the ash deposits and found small glass spherules—on average, about 50 μm in diameter—in ash deposits collected from both volcanos. Although the spherules were low in abundance, composing less than 5% of examined deposits from Mound Redoubt and even less from the Iceland samples (there scientists only identified two spherules), the findings suggest that glass balls can form from volcanic lighting.

Shifting correlation to causation, the scientists also mimicked volcanic lighting in high-voltage insulator experiments in the lab. This much safer, more controlled process also generated glass spherules, although these were much smaller with an average diameter of less than 20 μm.

0309ctt volcanic glass sphere lo res

Secondary electron image showing a glass spherule formed in high-voltage flashover experiments to examine the effect of ash contamination on electrical insulators. Credit: Kimberly Genareau

The researchers speculate that these spherules, called lightning-induced volcanic spherules (LIVS), form in the eruptive atmosphere from volcanic ash when it’s heated rapidly during lightning discharge. The intense heat of the discharge, which can reach temperatures of up to 30,000 K, melts and fuses the particles, which quickly cool and form glass.

While some of the glass spherules were smooth, others were cracked or contained holes that “appear to result from outward expansion of the spherule interior,” according to a Geological Society of America press release.

The spherules’ composition consisted of primarily silicon, with lesser amounts of aluminum, calcium, and iron, the authors report in the paper.

The open-access paper, published in Geology, is “Lightning-induced volcanic spherules” (DOi: 10.1130/G36255.1).